This invention relates to attenuators, particularly broad band, thin film attenuators for microwave applications.
In the construction of microwave circuits it is often desirable to employ an attenuator whose attenuation and input impedance remain constant from DC through the highest frequency that the circuit will experience. Microwave attenuators have been constructed as thin film devices; that is, devices employing a combination of flat conductors and resistive elements separated from a flat ground plane conductor by a thin, typically ceramic, insulating material. However, thin film microwave attenuators heretofore known have had some drawbacks. Typically, above an upper frequency limit their input impedance decreases significantly with increasing frequency. At the same time, their attenuation decreases significantly with increasing frequency.
One type of microwave attenuator that exhibits relatively constant attenuation to a relatively high frequency is a card attenuator of the type shown in Weinschel U.S. Pat. No. 3,157,846. However, such an attenuator also has some drawbacks that limit its usefulness. In particular, the electric field of the microwave signal in the resistive element is concentrated in that portion of the resistive element near the input conductor. As a result, that portion experiences high current density which limits the maximum power dissipation that the attenuator can provide, as excessive power dissipation will destroy the resistive element. Increasing the input contact area to increase power dissipation also increases the distributed capacitance, which lowers the upper frequency limit. Moreover, such a card attenuator employs a cylindrical shield surrounding a plate-like attenuation element and is therefore not physically convenient for all applications.
Accordingly, there is a need for a microwave attenuator, particularly of the thin film type, with improved bandwidth, input impedance characteristics, and power dissipation capability.